Traveling-wire electroerosion is a machining process which makes use of a thin, flexible, elongate, conductive element such as wire as the tool electrode for electroerosion of a conductive workpiece. Such an element in the process typically is 0.05 to 0.5 mm in thickness or diameter and is hereinafter referred to as "wire electrode" or "electrode wire" as commonly called in the art. The wire electrode is axially transported generally continuously from a supply means to a takeup means through a cutting zone in which the workpiece is disposed. The cutting zone is defined between a pair of guide members which support and hold the electrode wire while traveling through the workpiece. Wire traction and braking means allows the continuous wire to be stretched and generally taut under a given tension and to be axially driven between the guide members while traversing the workpiece, thus presenting a continuously renewed electrode surface juxtaposed in electroerosive cutting relationship with the workpiece across a narrow cutting gap. The cutting gap is flushed with a fluid medium and is electrically energized with a high-current density electrical machining current which is passed between the electrode wire and the workpiece to erode the latter, thus erosively removing material therefrom.
The cutting process may be performed in any of various electroerosive machining modes. In electrical discharge machining (EDM), the fluid medium is a dielectric liquid, e.g. deionized water, and the machining electric current is supplied in the form of a succession of electrical pulses. In electrochemical machining (ECM), the fluid medium is a liquid electrolyte, e.g. an aqueous electrolytic solution and the machining current is a high-amperage continuous or pulsed current. In electrochemical-discharge machining (ECDM), the fluid medium has both electrolytic and dielectric natures for the machining actions and the machining current is preferably supplied in the form of pulses which facilitate the production of electrical discharges through the conductive liquid medium.
The workpiece may be disposed in a bath of the liquid medium to immerse the cutting region therein. More typically, however, the cutting zone is disposed in the air environment. Advantageously, one or two nozzles are disposed at one or both sides of the workpiece to deliver the flushing medium along the electrode wire into the cutting zone disposed in the air environment or immersed in the generally static mass of the liquid medium. The cutting liquid medium is conveniently water as mentioned or an aqueous solution, which is deionized or ionized to a varying extent to serve as the desired particular electroerosive cutting medium.
To advance electroerosive material removal in the workpiece, the latter is typically displaced relative to the traveling wire and transverse thereto. This allows the traveling wire to advance translationally in the workpiece and consequently a narrow cutting slot is progressively formed behind the advancing wire, the slot having a width slightly greater than the wire thickness. The continuous relative displacement along a programmed path results in the formation of a cut contour corresponding to such a path and defined by this cutting slot in the workpiece.
Higher cutting speed in the process described is an ever increasing demand in the industry and is desirable to achieve with due precision. Thus, in order to achieve a maximum cutting rate defined in terms of volume of stock removed per unit time or, given a wire thickness, area of stock removal per unit time, and thus to minimize the cutting time, the cutting feed rate (i.e. rate of relative displacement between the wire and the workpiece along the programmed path) may be preset at a maximum value and/or servo-controlled, while other cutting parameters are adequately preset and/or in-process controlled, so as to achieve such a maximum value.
As mentioned previously, the guide members at the opposite sides of the workpiece are provided to establish a straight-line path for the span of the wire traveling through the cutting zone and a considerable tension is applied to the traveling wire across these guide members in order to maintain its axis across the cutting zone in alignment with that path. As the cutting feed rate is increased, however, it has been found that the traveling span of thin wire bends or flexes backwards or in the direction opposite to the cutting direction due to an erosion-machining pressure created in the narrow gap. This results in a cutting inaccuracy, i.e. a deviation of the cut shape of contour from the commanded shape, especially where cutting proceeds through an arc or corner. Furthermore, when flexing exceeds a certain limit, the wire may break. This phenomenon or the wire flexing which seriously affects the cutting accuracy is not properly ascertained or overcome by a usual servo system designed to control the cutting feed in response to a gap voltage or current inasmuch as the cutting front of the workpiece remains equally spaced from the bent wire electrode and there is essentially no sensible change from a normal gap voltage or current.